[Update]: The spell-focused Strixhaven expansion has now arrived, and as expected, the draft format is delightful!

In a classic best-of-three showdown, the 15-card sideboard serves as a way to adjust your strategy between matches and hopefully sure up any weaknesses you might have to the opponent's combo. Because of this sideboards in competitive Magic: The Gathering are incredibly important, and knowing which cards to include and when to bring them out easily be the difference between a sure defeat and a crushing victory.

The reason I'm mentioning this is because the upcoming Strixhaven expansion will bring with it plenty of cards with the "Learn" keyword that allow you to pull "Lesson" type cards from the sideboard. Since your sideboard is limited to a mere 15 cards, including these Lessons has an enormous deckbuilding cost as you sacrifice flexibility between games for more options within a single match.

However, when it comes to MTG Arena's best-of-one formats, any cards that draw from the sideboard are essentially free of this additional constraint since there is no sideboarding process between games. So in order to ensure nothing goes haywire with Strixhaven's release, Wizards of the Coast has now decided to preemptively nerf Lessons by reducing the best-of-one sideboard from 15 to 7 cards.

While Lessons themselves don't look powerful enough to warp the format, even if they do come as a freebie, I also remember how obnoxious it was to deal with adventure-based decks that would just endlessly spawn perfect answers through the use of Fae of Wishes. As such, I can't help but agree with Wizards of the Coast's decision here. There should be a real cost to using the sideboard as an extra deck, be that in best-of-three formats or best-of-one, and the size reduction does a pretty good job of making that so.

You can read the full explanation behind this change, as well as find out what else is coming with Strixhaven, over at the MTG website.